The younger of the two brothers accused of the Boston Marathon bombings has said that the pair were inspired by watching online videos of Anwar al-Awlaki, an American citizen who was killed by a U.S. drone strike.

Dzhokhar Tsarnaev, 19, told investigators that he and his older brother Tamerlan were motivated in part by the radical Muslim cleric who was killed by the U.S. in Yemen on September 30, 2011, reports The Wall Street Journal.

U.S. government officials said that al-Awlaki was a senior talent recruiter and motivator who was involved in planning terrorist operations for al-Qaida. He reportedly played a role in the radicalization of the so-called underwear bomber Umar Farouk Abdulmutallab of Nigeria, who pleaded guilty of trying to blow up an American-bound airplane on Christmas Day, 2009.

Al-Awlaki had many of his videos posted on YouTube until they were removed following a request from Congress in November 2010. According to CNN, however, the Dagestan wing of the Caucasus Emirate, a jihadist group fighting to create an Islamic state in the region, also posted videos of al-Awlaki.

The Tsarnaev brothers, of Chechen origin, spent much of their childhood in Dagestan before emigrating to the U.S. with their family in the early 2000s. Investigators, however, have reportedly not found evidence of direct contact between the Tsarnaev brothers and the cleric.

They are still trying to determine whether the brothers carried out their alleged attack on their own or had help from others. Of particular interest is a six month trip that Tamerlan made to Russia in 2012. That came after Russian law enforcement officials told their American counterparts that they suspected him of having become radicalized.

Tamerlan Tsarnaev, 26, was killed in a shootout with police on April 18, three days after the bombings, while Dzhokhar Tsarnaev, who was badly wounded in the exchange of gunfire, was captured the next day. He is currently in a prison hospital at Fort Devens, Mass.

Tsarnaev told interrogators that he and his older brother had considered carrying out an attack on July 4 but instead chose April 15, Patriots’ Day in Mass., because the bombs were ready earlier than expected, according to several reports.

He said the brothers built their pressure-cooker bombs in the older brother’s apartment in Cambridge, Mass., which he shared with his wife, Katherine Russell, and their child.

Female DNA and fingerprints found on a bomb fragment recovered from the attacks do not match the widow, reports CBS News. The FBI collected DNA and hair samples from Russell at her parents’ home in North Kingstown, Rhode Island on Monday.

Russell is not a suspect and has not been charged, but investigators want to question her further to find out what, if anything, she knew about her husband’s plans.

On Wednesday, prosecutors charged two 19-year-old college friends of the younger brother, Dias Kadyrbayev and Axamat Tazhayakov, both from Kazakhstan, with helping to hide evidence of the bombs by discarding a backpack containing gutted fireworks. They also charged a third friend, 19-year-old Robel Phillipos, with lying to prosecutors. None of the three has entered a plea.

According to an affidavit unsealed Wednesday, after seeing the widely released surveillance video of the suspects, Kadyrbayev sent his friend a text saying that he resembled one of them, to which Tsarnaev responded, "LoL, you better not text me."

He also texted Kadyrbayev to say "Come to my room and take whatever you want," said the affidavit, noting "Kadyrbayev knew when he saw the empty fireworks that Tsarnaev was involved in the marathon bombing. Kadyrbayev decided to remove the backpack from the room in order to help his friend Tsarnaev avoid trouble."